This invention relates generally to the field of munitions and ordnance, and more specifically to high velocity, kinetic energy projectiles that can penetrate deeply into the earth or hardened targets.
Large, high velocity kinetic energy penetrators are used as munitions, weapons, and vehicles to carry instrumentation or other apparatus. As such, they are typically delivered by aircraft, missiles, or cannon into the ground, a body of water, or a man-made structure, hereafter referred to as the target. These types of penetrators usually carry a payload of instrumentation or high explosive and must survive the violent actions that accompany impact and sudden deceleration, all the while protecting and preserving the payload. Examples are penetrators built to attack buried military targets surrounded by thick concrete ceilings and walls, or penetrators built to carry instrumentation to measure the geologic character of the earth or properties of arctic ice as they pass through it.
A penetrator can be subjected to both high positive and negative longitudinal acceleration forces, as well as rotational acceleration forces, during its brief flight. The device may be subjected to a positive acceleration on the order of 5000 g during launch by a missile or gun, and it may be subjected to a negative acceleration on the order of 20,000 g upon impact with a hardened target. Because of these loads, it is preferable that the case be a monolithic construction, i.e., formed from a single piece of hard material such as a high-strength steel alloy. The use of monolithic construction eliminates failures of joints and fasteners that are possible in multi-part cases. An example of a monolithic penetrator currently in use as an anti-tank weapon is the class of sub-caliber solid tungsten "spears" or "darts" that are conveyed by a sabot during gun launching.
Prior art penetrators have been used successfully at low velocities against hard targets such as competent rock and concrete, or at high velocities against soft targets such as soil. Designing penetrators that can penetrate deeply and survive the impact with hard targets at velocities in excess of 2000 feet per second (ft/s) has been found to be particularly difficult, especially for sizes larger than the small prototypes used in indoor laboratory testing. The present invention can impact the media and survive at velocities up to and exceeding 4000 ft/s. High velocity impacts with hard targets can cause severe nose abrasion, bending, and frequent breakage. Penetration depth is reduced in hard targets. Also, the high deceleration forces that accompany impact can damage the payload.
Penetration of hard targets is achieved by concentrating a high amount of kinetic energy (KE) on a small area to create a very high stress. Use of heavy metal penetrators, such as tungsten (which has a density about twice that of steel) allows the KE to be doubled while keeping the outer dimensions of the penetrator constant, thereby penetrating the target to a much greater depth. These penetrators are typically pointed bodies fabricated in the shape of a "spear" or a "dart", often with guiding fins, from sintered tungsten or liquid-phase sintered W--Ni--Fe alloys. These "dart" type penetrators are typically sub-caliber and require the use of a sabot holder during gun launch.
It is well known to those skilled in the art that if a means were found to increase the total mass while keeping the outer dimensions of the penetrator constant, then the shocks and decelerations of impact would be lessened and the penetrator would travel deeper into the target. Unfortunately, tungsten is brittle and fails prematurely when used to construct the penetrator. Lead, another dense, heavy metal, is much too soft and weak to be used for this application. The best use for heavy materials is as a ballast, while using high-strength steel for the penetrator's nose and case.
When a heavy material is used internally as ballast it is usually difficult to secure and hold in place during violent impacts without making use of large mechanical fixtures, which take up space desired for the payload. Because it is simply carried as a high-density mass, prior art ballasts typically contribute no strength to the penetrator's case. Rather, the ballast adds to the loads and forces that the penetrator's case must support in order to survive.
Prior art penetrators are typically made from steel or tungsten ingot or bar stock by a combination of forging and machining operations. Forging is used to reduce the ingot to nearly the proper diameter and sometimes to create a cavity in the interior that will become the payload bay. Machining then creates the final shape. If the penetrator is made of steel, then it must be heat-treated to achieve a hardness and strength necessary for survival during penetration of the hard target. At some point the ballast, if required, must be installed. Pure mechanical attachment by machined threads or bolts is difficult and expensive. High temperature joining methods, such as brazing or diffusion bonding, destroy the prior heat treatment of the steel case and reduce its hardness and strength. Finally, it is desirable to minimize the cost of a penetrator by reducing the number of fabrication steps and the time necessary for expensive machining operations.
Another class of prior art penetrators utilize shaped explosive charges to create a hyper-velocity jet of molten metal which is very effective at penetrating thick (multi-inch) metal armor (such as tank armor). However, these devices do not perform well against massively thick concrete bunkers (.about.10 feet thick), and typically are more expensive and complex to manufacture than simple heavy metal penetrators.
Another class of prior art penetrators utilizes a single, heavy metal "dart", or blunt or pointed rod that is contained inside of a hollow steel case. The heavy rod is released from the case upon impact and travels alone through the target. Often, the nose of the projectile is made of a hollow, thin-walled ballistic shroud. The present invention differs from this design in that the present heavy material ballast remains completely contained inside of, and travels with, the steel case during penetration.
Most prior art penetrators do not carry a payload.